The present invention relates generally to film thickness measurement systems, and more particularly, to film thickness measurement systems for use with structures containing a scattering surface.
In the fabrication of simple silicon wafers and bonded silicon wafers, there is a requirement to measure layer thicknesses at an early stage during manufacture when only one side of the wafer is polished, in the case of a single wafer, or when the top surface of a bonded wafer is mechanically polished to a level where plasma polishing can start. Bonded silicon wafers may be measured using the visible region of the optical spectrum since the silicon layers are thin, whereas thick single wafers must be measured using the infrared region of the optical spectrum due to high light absorption levels at visible wavelengths.
If a thin film silicon wafer is illuminated with narrow band radiation, optical fringes are formed across the aperture, although they have reduced finesse or contrast due to averaging effects introduced by an exposed scattering surface (the top, fine ground scattering surface). The coupling between the structured layer and the remaining thin films is usually coherent in this case since film thicknesses are less than the coherence length of the light source. A thickness measurement may be made under these circumstances if a library used for pattern matching includes the effects of the scattering or surface structure details. In the simplest case of a ground surface that forms the outer or scattering surface, a model may be created based upon the random film thicknesses caused by the grinding process and the large range of angles, and hence optical path differences that occur in the layer.
Typically, scatterplates and scattering surfaces have been used in conventional interferometric and holographic measurements of mirror or object surfaces. However, these conventional approaches measure the surface profile of the mirror and not the thickness of a thin film disposed on the surface of the mirror. Prior art exists for the case where structured surfaces consisting of diffraction gratings are buried inside or placed on top of multilayer coatings. These devices have been used as dichroic reflecting beamsplitters and beam sampling optical elements in high energy laser systems. However, there is no prior art regarding the use of scattering surfaces that are coherently coupled to a thin film structure to measure partially finished wafers, or the use of a scattering surface as a beamsplitter (incoherent coupling) to allow multispectral imaging of large wavy reflecting surfaces.
Thickness measurements of thin films on silicon, including bonded silicon-on-insulator (SOI) wafers comprising planar films are currently measured by recording full aperture fringe patterns at numerous wavelengths and deriving the spectral characteristics of the film over the entire wafer. This data is then pattern-matched against a precomputed library to determine the best fit spectrum, and hence the film thickness of one of the layers. In this case, the library models the reflectance spectral patterns of plane parallel, homogeneous layers as found in conventional thin film structures.
Therefore it is an objective of the present invention to provide for film thickness measurement systems for use with structures containing a scattering surface. It is also an objective of the present invention to provide for film thickness measurement systems that use scattering surfaces that are coherently coupled to a thin film structure to measure partially finished wafers, and use a scattering surface as a beamsplitter (incoherent coupling) to allow multispectral imaging of large wavy reflecting surfaces.